Liquid crystal displays (LCD) that are bistable under zero voltage bias conditions are desirable for many applications. There are many LCD configurations that are bistable and possess two stable states under no voltage conditions. Examples include: (1) bistable cholesteric LCD; (2) bistable ferroelectric LCD; (3) bistable twisted nematic LCD where the two stable twist states differ by 360 degrees in twist angle; (4) bistable twisted nematic LCD where the two stable twist states differ by 180 degrees in twist angle; and (5) zenithal bistable display, which is based also on nematic liquid crystals. Each of these bistable displays has its own merits and shortcomings.
The present invention is related to the two types of bistable twisted nematic LCD displays listed above. Both classes of bistable liquid crystal displays are based on the twisted nematic effect in a liquid crystal display. They are based on the interplay between the elasticity of the liquid crystal and the surface anchoring conditions. The 360 degree bistable nematic LCD have bistable states for both twist states. In the 2π bistable twisted nematic liquid crystal display, the bistable twist states are zero twist and 360° twist states (see, D. W. Berreman and W. R. Heffner: J. Appl. Phys. 52 (1981) 3032; and D. W. Berreman: J. Opt. Soc. Am. 63 (1973) 1374.). Kwok teaches a generalization of such bistable twisted nematic displays where the bistable twist states are φ and φ+2π twist states where φ can be several fixed values, both negative and positive (see, H. S. Kwok: J. Appl. Phys. 80 (1996) 3687, T. Z. Qian, Z. L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett. 71 (1997) 596, Z. L. Xie and H. S. Kwok: Jpn. J. Appl. Phys. 37 (1998) 2572, and Z. L. Xie and H. S. Kwok: J. Appl. Phys. 84 (1998) 77). These φ values have been calculated and experimentally verified. These bistable twisted nematic displays can be called 2π-BTN displays.
Durand teaches another variant of the 2π-BTN bistable twisted nematic display where the bistable twist states are zero and 180° twist states (See, I. Dozov, M. Nobili and G. Durand: Appl. Phys. Lett. 70 (1997) 1179.). The switching of such a display is more difficult (but not impossible), than the 2π-BTN bistable liquid crystal displays. Strong asymmetric anchoring and thin cell gaps have to be used. Again, Kwok teaches a generalization of such bistable twisted nematic displays where the bistable twist states are φ and φ+π twist states, and where φ can be one of several published values (See H. S. Kwok: J. Appl. Phys. 80 (1996) 3687, T. Z. Qian, Z. L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett. 71 (1997) 596, Z. L. Xie and H. S. Kwok: Jpn. J. Appl. Phys. 37 (1998) 2572, and Z. L. Xie and H. S. Kwok: J. Appl. Phys. 84 (1998) 77.). Thus, another name for this class of bistable twisted nematic LCD is π-BTN bistable liquid crystal displays.
This invention relates to a method of construction and a driving scheme for the π-BTN bistable twisted nematic LCD. In this invention, a special combination of alignment layers and liquid crystals guarantees that bistability is achieved readily and that the driving scheme enables this bistable display to be switched easily. In particular, the cell gap can be much larger than the 2 μm required for the Dozov π-BTN. As well, the driving scheme is much simpler.